Seasonal compensation of microbial production and respiration in a temperate sea

Gross oxygen production (GP), dark respiration (DR) and net community produ ction (NCP) were studied for 16 mo in the euphotic layer of 3 stations thro ugh the coastal transitional zone of the southern Bay of Biscay, and relate d to hydrographic and nutrient conditions, phytoplankton biomass and C inco rporation. Microbial O-2 fluxes exhibited seasonal patterns linked to the s easonal cycle of water column stratification and mixing, with positive NCP during the spring, negative throughout the summer and close to zero in wint er. This pattern was altered at coastal regions, where productive periods w ere linked to coastal upwelling, whereas in winter persistent net heterotro phy was measured, presumably in relation to increases in organic matter dis charge of continental origin. The comparison of NCP with O-2 anomaly and NO 3 concentration in the euphotic zone, the spatial and temporal scales studi ed and the prevalence of steady-state conditions offshore support the concl usion that the maintenance of summer heterotrophy in the region was based u pon the consumption of the surplus of organic matter produced in spring. Th e uncoupling in the microbial auto- and heterotrophic metabolisms, based on the accumulation and delayed consumption of dissolved organic matter as a consequence of the processes controlling phytoplankton growth and microbial heterotrophic activity in temperate seas, would explain such a pattern. Th e close relationship observed between the seasonal variability in NCP and t he magnitude of spring net production and predictions derived from the seas onal cycles of O-2 anomaly in middle latitudes and atmospheric O-2 led us t o conclude that the seasonal compensation of production and respiration pro cesses is a characteristic of the dynamics of the pelagic ecosystem, at lea st in coastal temperate seas. The implications of this conclusion are of gr eat relevance for the interpretation of new production and the estimation o f the trophic status of the ocean from direct measurements of plankton net production.